Geospeedometry

Today whilst reading my 1000 words from the geologic literature (61 days in a row this time, and counting) I encountered a term that either I have never seen before, or I somehow managed to overlook it on other occasions. “Geospeedometry” was coined by Lasaga (1983), who related the cooling rate calculated for minerals based upon the diffusion of atoms within minerals with the time it takes for the “exhumation” of the rocks within which the minerals grew. What does this mean?

Metamorphic rocks form when any preexisting rock is subjected to increased temperatures and/or pressures for sufficient time to grow new minerals which are stable at the new conditions. One very common way for this to occur is for the rock to be taken sufficiently deep below the surface of the earth that both the temperature and pressure are elevated. If it were to happen that a package of rocks were to be taken to such pressures and temperatures and held there until all new minerals grew to replace the original minerals, and then those rocks were to be very slowly brought to the surface so that new minerals continued to grow to replace older minerals during the changing conditions the ultimate result would be a rock which contains only minerals which are stable at surface conditions. However, it happens often that the metamorphic rocks containing minerals which grew at elevated pressure and temperatures are brought back to the surface too quickly for those minerals to be replaced by their lower pressure/temperature counterparts. As a result we have a record of the conditions at which the metamorphism happened. The process of bringing the rocks back to the surface is called “exhumation”, and it refers to great quantities of over-lying rock going away (often due to a combination of faults bring up underlying rocks, and erosion carrying away broken bits of overlying rocks).

Ever since geologists realized that each mineral has a specific range of temperatures and pressures at which it will grow people have been attempting to figure out how to relate the list of minerals present in a given rock with the temperature and pressure at which it formed. The next logical question after the conditions of formation have been determined is one of “how long”. How long did the minerals take to grow? How long (or how quickly) did it take to get this rock from where it formed to the surface of the earth? Those people who study compositional zoning in minerals and calculate the rate of diffusion of atoms within the minerals and who then relate those numbers to the time it took for the diffusion to occur describe what they are doing as “geospeedometry”. Since the term was coined in 1983 there have been 52 papers which list that term in their title, abstract, or key words that have been entered into the Scopus database. One each published in 1983 and 1984, and then a six year break before the next was published. Since 1990 there have been one to five papers on geospeedometry published a year, save for 1993, which didn’t have any.

It is interesting to me that even after completing a PhD and making a point to try to read papers from the geologic literature on a daily basis, I am still encountering terms that are new to me, though they have been around for decades.

Lasaga AC. 1983. Geospeedometry: an extension of geothermometry. In Kinetics and Equilibrium in Mineral Reactions, ed. SK Saxena, Adv. Phys. Geochem., 2:81–114. Berlin: Springer-Verlag

nano particles

About five years ago, I read the book The Diamond Age. This was my first introduction to the concept of “nanotechnology”, and, I must confess, that I paid it scant attention. There being complicating factors in my life at the time (having just lost my beloved step-father to cancer), I accepted the work of fiction as a distraction from the world around me, letting the wonders of that world exist solely in that world, and not making time to consider to what level, if any, such technology might exist in the real world. Nor did I put any thought into just how small “nano” might be.

Today’s mail brought me the latest issue of Elements Magazine. which focuses upon nanogeoscience, with articles on the history of our understanding of the nano scale and how technology is being applied to study things that small, of the changes in behaviour of minerals and elements at the nano scale (16% of a 10 nm cube’s atoms are near the surface of the cube, which can have profound effects upon the way that cube participates in chemical reactions when compared to a 10 μm or even 10 mm cube), and more.

But how small is a nano particle? “Nano” as a prefix in the metric scale means 10^-9, so 1 nm is 1000 times smaller than a micrometer (μm), which is 1000 times smaller than a millimeter (mm), which is a thousand times smaller than a meter (m). The example given in the article to help us understand what this really means was to compare our planet, Earth, to a standard light-bulb. Apparently Earth is about as much larger than a light-bulb as the light-bulb is larger than a single nanometer.

One of my favorite “toys”, I’ve been privileged to play with as part of my PhD research is the electron microprobe, which lets me see some amazing detail in the crystals in my rock samples. With this tool I may analyze the composition of minerals which are (so I thought) quite small—so long as they are at least 10 μm wide I can be reasonably confidant of analyzing that mineral and not the neighbouring ones. Other scientists out there are using different technology to look at things so much smaller than my tiny minerals that the scale bars in some of the photos accompanying the articles are only 20 nm long.

I have heard it said that any technology which is sufficiently advanced appears to be magic to those who do not understand it. When I read that work of fiction all those years ago, I accepted the descriptions of the “nanotechnology” as the “magic” inherent to that world, and thought nothing more about it. Reading the articles today which tie the concepts of nanotechnology with the geosciences and the minerals with which I’m interacting on a daily basis somehow brings it home to me in ways that I wasn’t ready to think about five years ago. You can bet that I shall be paying attention the next time I encounter the concept, be it in fiction, on line, or even in the local newspaper.